Lifting-off of epitaxial layers is a process used in the semiconductor industry or other nano-technology related industries. For example, semiconductor materials can be in the form of epitaxial layer materials suitable for a range of potential applications including ultraviolet to visible optoelectronics (e.g. LEDs and lasers) and high temperature electronics (e.g. transistors). Amongst such semiconductor materials, Group III-V nitride semiconductor materials include aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) and their related ternary and quaternary alloys such as aluminum gallium nitride (AlGaN) and indium gallium nitride (InGaN).
Among Group III-V nitride based materials, GaN, a direct-bandgap semiconductor material of wurtzite crystal structure with a wide (3.4 eV) band gap, is currently considered as the prominent semiconductor for applications in optoelectronics (e.g. LEDs from UV to green-blue and white for solid state lighting, laser diodes), and high power and high frequency electronic devices (e.g. High Electron Mobility Transistors (HEMT)). Its sensitivity to ionizing radiation is low, making it a suitable material for solar cell arrays for satellites. Furthermore, since GaN transistors can operate at high temperatures and high voltages, they make ideal power amplifiers at microwave frequencies.
GaN has the advantages of being mechanically hard and chemically inert. However, due to the high melting temperature and the high equilibrium vapor pressure of nitrogen (N2) at the growth temperature to synthesize III-V nitrides, large bulk single crystals for homoepitaxy are costly to produce in high temperature, high pressure conditions and are currently limited to 2 inch wafers. Crystalline GaN is usually grown epitaxially on substrates of dissimilar materials. Silicon (Si), silicon carbide (SiC) and sapphire are the most commonly used substrates, and GaN films are deposited via methods such as, but are not limited to, metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or hydride vapour phase epitaxy (HVPE). Despite its widespread use, the physical characteristics of sapphire, such as being electrical insulating and its relatively poor thermal conductivity, render it unsuitable for device fabrication. In addition, although these substrates are used for commercial products, there are still issues which degrade the quality of epilayers grown on them. Typically, the epilayers exhibit a crystalline defect level of around 108˜109 cm−2 due to lattice and thermal mismatches. This high level of crystalline defects is an obstacle to device performance. For example, it leads to low internal light emission efficiency and short life times.
Moreover, besides epilayer quality, bulk substrate properties are far from optimal from a device performance point of view. They inhibit device performance due to their poor thermal and electrical conductivity. The poor thermal conductivity of these substrates prevents efficient dissipation of heat generated by GaN-based high-current devices, such as laser diodes and high-power transistors, which consequently inhibits performance of devices. The poor electrical conductivity of these foreign substrates, especially sapphire, complicates the contact and packaging schemes, resulting in spreading resistance disadvantages and higher voltages for device operation.
One removal technique of a GaN epilayer structure from the sapphire substrate involves the use of a laser to lift off the GaN film that is epitaxially grown on a sapphire substrate. A laser beam irradiates the epilayer through the backside of the substrate, which locally heats the epilayer near the substrate interface and decomposes the epilayer into its constituents, Ga metal and nitrogen gas. After irradiation, the epilayer and the substrate can be separated by heating above the melting point of Ga metal of 30 degrees Celsius. As both GaN and sapphire are transparent at visible wavelengths, an intense lightbeam in the UV wavelength range is required, which can only be produced by an expensive high power laser, such as an excimer laser. The limited beam spot size of a laser means that the beam has to be scanned across a relatively large application area, which can generate transient spatial nonuniformities in heating and thermal expansion across the wafer, thereby cracking the epilayer during laser lift-off. In addition, the relatively expensive laser equipment and short life-time of the laser equipment with low production efficiency render this technique inappropriate for large quantity production. However, it is reported that high energy laser treatment can also cause surface roughness and interdiffusion of aluminum and oxygen into GaN and post polishing is usually required to achieve the desired surface roughness and film thickness. Furthermore, the high energy laser in the process increases the cost of the product.
A second removal method of a GaN epilayer structure from the sapphire substrate is the technique of Epitaxial Lift-Off, which involves the use of a sacrificial layer to be disposed between the epilayer layer and the substrate. A typical sacrificial layer is made of a compound that is chemically distinct from the remaining layers and which can be selectively etched, removed or decomposed, thereby releasing the GaN epilayer structure from the growth substrate.
The ELO method has the following disadvantages. Firstly, it requires the use of a material system that is compatible with the epitaxial growth of GaN, and which can be selectively removed by chemical etching. While there are some reports of using sacrificial layers, for example, GaN/ZnO (as sacrificial layer)/sapphire, or GaN/CrN (as sacrificial layer)/sapphire, however, the material quality is far less optimized. Secondly, the formation of bubbles at the etch site due to a redox reaction during etch layer dissolution may cause the thin GaN layer above to warp and crack, which affects its electrical and optical characteristics. The use of electrochemical etching technique in ELO resolves the bubble formation issue, as reduction is carried out at a remote electrode (cathode).
One variation of the above method comprises the step of applying an electrochemical potential between the layered material/substrate and a counter electrode to oxidize and dissolve a thin etch layer positioned between the film and substrate, which frees the layered material from the substrate.
Another variation of this method is Photoelectrochemical (PEC) Etching, which makes use of a high power UV lamp to selectively excite a sacrificial layer within the GaN-on-sapphire epilayer structure, such that the sacrificial layer is electrochemically etched/dissolved by the electrolyte. Photo-excitation is needed to generate electron-hole pairs to participate in the reactions necessary for material removal.
However, the PEC processes reported to-date result in a rough etched interface with the formation of facet islands or whiskers, thus, post etching or polishing process is needed. And also, the electrolytes usually HCl or KOH used in the PEC etching attack the structure layers near the threading dislocations as well, resulting in a damaged lift-off film on the top surface. In addition, the etching selectivity between the sacrificial layer and structured layers is poor, which makes it difficult to lift-off of the electrically driven device structure with doped GaN and active layers in it.
Besides the concerns above regarding removal of substrate from a GaN epilayer structure, for LED applications, light tends to be trapped in the high-index semiconductor by total internal refraction, which is an issue with regards to light extraction. Considering the refractive indices of GaN (n=2.5) and air, the critical angle for the light escape cone is about 23°. Assuming that light emitted from sidewalls and backside is neglected, one expects that approximately only 4% of the internal light can be extracted from a surface. The light outside the escape cone is reflected into the substrate and is repeatedly reflected, then reabsorbed by active layers or electrode, unless it escapes through the sidewalls. Surface nanopatterning is one of the methods for improving the light extraction as the nanopatterned surface reduces internal light reflection and scatters the light outward.
A need therefore exists to provide a method of at least partially releasing an epitaxial layer that seeks to address at least one of the abovementioned problems.